The filtering apparatus for purifying water by filtering the pollutant in the polluted water is generally equipped with a porous membrane through which the polluted water passes. The pollutant in the polluted water is filtered by the porous membrane, by which the water passing through the porous membrane is discharged as a clean water.
The problem hardest to overcome in such a liquid-solid separation technique using the porous membrane is the abrupt declination of the liquid(or gas)-solid separation capacity of the separation membrane as the size of the pores that are the passages of the filtered liquid is reduced or the pores are blocked by the solid material adhered to the surface of the separation membrane or to the inner surface of the pores during the separation process. A variety of methods have been proposed in the past several decades in order to solve such a problem.
U.S. Pat. No. 3,437,208, Apparatus for Dynamic Filtration for Liquids, has proposed the structure that a rotary type (or fixed type) disks having blades are disposed between the piled fixed type (or rotary type) separation membranes and are rotated, thereby preventing the declination of shear intensity for the separation membranes by generating shear force for detaching the pollutant adhered to the surface of the separation membranes.
U.S. Pat. No. 4,036,759, Apparatus and System for Stabilizing the Disk element of a rotary concentrator for solids containing fluids, discloses the structure that a shoe is so mounted on the rotating part, that is, on the outer circumferential surface of the supporting plate of the rotary type disk or the rotary type separation membrane, as to rotate along the guide recess of a housing. According to such a construction, the problem occurring in the structure that the rotary type (or fixed type) disks are disposed between the piled fixed type (or rotary type) separation membranes as in the U.S. Pat. No. 3,437,208, that is, the deformation and the displacement in the shaft direction of the disk due to the pressure difference between both surfaces of the disk are prevented and therefore the stability of the system increases.
U.S. Pat. No. 5,275,725, Flat separation membrane leaf and rotary separation apparatus containing flat membranes, discloses the structure that a fixed type partitions made of flexible material are disposed between the piled rotary type separation membrane units to prevent the deformation by the pressure difference and the breakdown of the separation membranes caused by the same.
U.S. Pat. No. 5,415,781, Dynamic filter separator and separation device, and U.S. Pat. No. 5,679,245, Dynamic filter system, disclose the structure of separation apparatus having the fixed type separation membranes and the rotary type disks with blades.
In such conventional filtering apparatuses, as mentioned above, the disks are disposed between the separation membranes in order to reduce the adhesion of solid material on the surface of the membranes by generating strong shear rate on the surface of the separation membranes through the relative movement between the separation membranes and the disks. However, the shear rate on the surface of the membrane by the relative movement decreases seriously as the distance between the separation membrane and the disk becomes great. If the distance between the separation membrane and the disk becomes small in order to increase the shear rate, the separation membrane and the disk may contact with each other by the pressure difference between both sides of the disk to cause the damage on the membrane, so the precise treatment and accurate assembly are required to prevent such a problem, which may cause the increase of the manufacturing costs. Furthermore, the pressure decrease occurs as the fluid flows along the long passage formed by the piled separation membrane—disk—separation membrane structure, and the fluid has to be supplied with greater pressure in order to maintain proper filtering pressure and prevent the decrease of performance by compensating such a pressure decrease. However, that causes the increase of the driving costs and management costs, which deteriorates the economical performance of the system.
U.S. Pat. No. 6,165,365, Shear localized filtration system, and U.S. Pat. No. 6,416,666, Simplified filtration system, disclose the technique that the centrifugal force and the rotational force are applied to the fluid with the viscosity of the fluid by rotating the piled separation membranes. According to that, the movement of the fluid between the separation membranes is caused to reduce the adhesion of solid material on the surface of the membranes. Furthermore, four through sixteen, optimally eight, fixed type spokes are disposed radially between the separation membranes, which makes the pressure distribution uniform and the speed of fluid between the spokes and the membranes great to increase the shear intensity, thereby preventing the adhesion of the solid material.
The above patent describes that the spokes promote the turbulent flow phenomenon at the surface of the membranes. However, it is considered that the effect of inducing the turbulent flow is quite little, since the flow in the circumferential direction and the radial direction is laminar flow. Therefore, the spokes in the above patent only has the effects that the uniform pressure distribution is achieved in the filter pack, and the adhesion of the solid material is minimized due to the change of the speed of fluid at the surface of the membrane by the change of the volume in the space from the separation membrane.
As mentioned above, in order to minimize the adhesion of the solid material at the surface of the separation membranes, it is the best method to increase the shear rate by vitalizing the flow around the separation membranes. However, the increase of the shear rate of the fluid at the surface of the membranes merely with the change of the speed of fluid by the relative movement of separation membrane—disk—separation membrane, or separation membrane—spoke—separation membrane construction in the conventional art proposed under such a purpose is limited.
SE 451429 and SE 459475 disclose the separation apparatus having separation membrane—rotor—separation membrane construction which is different from the above separation membrane—disk—separation membrane, or separation membrane—spoke—separation membrane construction. In those patents, the rotor is shaped into not the disk but a bar, so the rotation of the rotor causes not only the shear flow but also the turbulent flow between the separation membranes. It provides low loss of pressure since the passage between the membranes is narrow in comparison with the system having the disk type rotor, and furthermore, the bar shape rotor proposed in those patents has great influence on prevention of adhesion of solid material. However, that effect is not sufficient in fact, so the regeneration process for the separation membranes has to be performed regularly.
According to the above-mentioned SE 451429, the regeneration process for the separation membranes is the process that a mechanical element such as a brush or a valve is attached on the blade of the rotor and the material adhered to the surface of the separation membrane is removed by rotating it, which has the shortcoming that the porous coating on the surface of the separation membrane is also removed during that process. In order to compensate such a shortcoming, the surface of the membrane is newly coated, however, such a mechanical separation membrane regeneration process cannot maintain the required size of the pores as desired, and the separation membrane has to be exchanged with a new one when the regeneration is not easy. The above-mentioned SE 459475 proposes the method to increase the capacity by piling up the filter units.
U.S. Pat. No. 6,027,656 proposes a separation device that does not require the mechanical regeneration process since the stronger turbulent flow is induced between the membranes with the rotor of which shape is modified from the bar shape rotor. However, stronger turbulent flow is not expected since the employed rotor merely consists of two blades. Furthermore, the speed of the rotor is a sole factor in controlling the magnitude of the turbulent flow according to the kind or status of the fluid to be processed, so it is very hard to separate various kinds of liquid of various characteristics. In order to compensate such a shortcoming, the above-mentioned patent has proposed a method to equip an ultrasonic wave or electric field generation apparatus together with the rotors having various cross sectional shapes.
As describes so far, the most effective method for preventing the most significant problem, the adhesion of foreign substance on the surface of the membranes, in the liquid-liquid or liquid-solid separation apparatus using the separation membranes is to make the shear stress at the surface of the membranes as great as possible. It is necessary to generate the turbulent flow for such a purpose, however, the method that has been proposed so far can generate the turbulent flow within a limited range. In particular, the unit in U.S. Pat. No. 6,027,653, which is expected to induce the stronger turbulent flow than the filter unit of rotary type separation membrane—fixed type spoke construction in U.S. Pat. No. 6,165,365 or than the filter unit of separation membrane—disk—separation membrane construction, employs the rotor having only two blades, so the turbulent flow is generated locally and the rotational speed has to be greater for the stronger turbulent flow.
Furthermore, although the fluid of different characteristics about the density, viscosity, etc. requires turbulent flows of different strength, the turbulent flow of desired magnitude can be achieved only by the change of the rotational speed of the rotor since the shape of the rotor is fixed. Therefore, the rotational speed has to be greater to generate the stronger turbulent flow regarding to the fluid of greater viscosity and density, which causes the increase of the required driving energy and the loss of the energy.